Not gonna engage much in speculation about weight, as I'm just not that interested. Yes, he appears to be quite skinny, as do most people running 2:06.
But I did check out Hirabayashi's IAAF page - this guy just turned 21 in December, and has listed PR's of 14:00 / 27:55 / 63:02 / 2:06:18. I'm surprised this performance isn't getting more coverage, 2:06 at age 21 seems like a massive performance; with any improvement over the next few years this guy could be a global contender. Just for reference, Kiyoto is about 6 months younger than Nico Young.
Kissa was a close second place; he's 35 and ran 2:04:48 NR for Uganda two years ago in Hamburg.